A few persons have said that since Vatican II, Protestants are advised by the Catholic Church to remain in their denominations, not convert to Catholicism. Is this true? Any document references would be helpful.
Setting the pattern for much more to follow, Unitatis Redintegratio announced a whole new "ecumenism" project being mandated, and was quite explicit in making clear the distinction between this new program of "ecumenism" versus what evangelization is. In paragraph 4 it states, "However, it is evident that, when individuals wish for full Catholic communion, their preparation and reconciliation is an undertaking which of its nature is distinct from ecumenical action."
So therefore every bit of "ecumenism" (Vatican II style) is quite specifically NOT about evangelization, and therefore about having connections (projects of mutual trust, interest, whatever) with non-Catholics without any attempt to convert them. In the "spirit" of such "ecumenism" a schismatic East Orthodox bishop wanted to convert, but was told by the Vatican that he was more useful as a "friendly Orthodox" than as a "fellow Catholic" and refused entry. And perhaps you might remember that "joint statement" issued in conjunction with the Lutherans (except those of the Missouri Synod, who commendably rejected the blatant insincerity of the project) some years ago.
This was not about Protestants being perhaps found to be right (or even Catholics for that matter, since by the time of that "joint statement" what they had come to call "Catholic" had no more connection with the Church that condemned Luther than the Lutherans ever had) but about deciding that spiritual truth is optional, as the same goes for Jews, Moslems, Hindus, Buddhists, Wicca, Snake worship, and so forth.
While this non-conversion "ecumenism" has been woven through many "official acts" too numerous to enumerate, I must admit that such things are not what I collect or have immediately on hand, though other researchers might have more of that, but at least here you get the bedrock, how it all started at Vatican II, and at least one other example.
If you need more examples, perhaps Fr. Michael, another Allexpert, can provide you with more examples. Hope this helps, God Bless!